It's not quite Shakespearean wordplay, but a species of African monkey can modify individual warning calls to produce novel meanings, according to new research. And because the wild monkeys tack the same sound onto the end of their calls, the authors speculate that they could resemble suffixes. But it's debatable whether the sounds serve a grammatical purpose like that in human language.

Campbell's monkeys, noted for their grizzled faces and tuft of orange fur on top of their heads, live in the dense rainforest of Western Africa. Like many species of monkeys, they use various warning calls: "Hok"[2] is for eagles, "krak"[3] is for leopards, and "boom" is for nonpredatory disturbances, such as a branch falling from a tree. The monkeys can even combine different calls to form new messages. For example, to signal less-dangerous situations, Campbell's monkeys holler "boom-boom" followed by "krak" or "hok." This tells other monkeys that the predator is far away, so there is no immediate need to flee. But scientists were skeptical that monkeys could modify individual calls, because the primates appear to lack the appropriate anatomy for sophisticated vocalization.

To find out how rich the vocal repertoire of monkeys really is, Alban Lemasson, a primatologist at the University of Rennes 1 in France, and his colleagues traveled to the Ivory Coast and monitored the alarm calls of Campbell's monkeys for 2 years. They found that monkeys have six alarm calls, twice as many as previously thought. The primates doubled their call repertoire by adding "oo" to their specific predator calls. "Hok-oo"[4] and "wak-oo" became a general alert for a canopy disturbance, whether an eagle or a flying squirrel, and "krak-oo"[5] signaled almost any disturbance. This trick allows monkeys to sidestep their limited vocal range and produce more messages, the team reported[6] last month in PLoS ONE. "The monkeys used a suffix system to create new calls," Lemasson says. "To my knowledge, this has never been shown before in animal communication."

The study suggests that the ability to modify words to produce new meanings evolved independently from human language, Lemasson says. He speculates that the density of the forest habitat puts selection pressure on Campbell's monkeys to develop a richer set of vocalizations, because it is challenging to communicate visually. "I'm sure that if we continue working with these species, we will discover additional vocal abilities," he says.

Evolutionary biologist Thore Bergman of the University of Michigan, Ann Arbor, says the work is promising but that it's unclear how analogous the monkey-call modifications are to human suffix usage. To test this, he says, scientists will have to show that monkeys react as if they understand the different meanings of the calls when scientists play back the alarm sounds, with and without the suffix. If the findings hold up, he says, it will be an important advance in understanding how our own language abilities evolved. "We're running out of things that make human language unique."